VPC3242D Просмотр технического описания (PDF) - Micronas

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VPC3242D

Comb Filter Video Processor

Micronas 

VPC 323xD, VPC 324xD

ADVANCE INFORMATION

2.3.1. IF-Compensation

With off-air or mistuned reception, any attenuation at

higher frequencies or asymmetry around the color

subcarrier is compensated. Four different settings of

the IF-compensation are possible (see Fig. 2–3):

– flat (no compensation)

– 6 dB/octave

– 12 dB/octave

– 10 dB/MHz

The last setting gives a very large boost to high fre-

quencies. It is provided for SECAM signals that are

decoded using a SAW filter specified originally for the

PAL standard.

dB

MHz

Fig. 2–3: Frequency response of chroma IF-com-

pensation

2.3.2. Demodulator

The entire signal (which might still contain luma) is

quadrature-mixed to the baseband. The mixing fre-

quency is equal to the subcarrier for PAL and NTSC,

thus achieving the chroma demodulation. For SECAM,

the mixing frequency is 4.286 MHz giving the quadra-

ture baseband components of the FM modulated

chroma. After the mixer, a lowpass filter selects the

chroma components; a downsampling stage converts

the color difference signals to a multiplexed half rate

data stream.

The subcarrier frequency in the demodulator is gener-

ated by direct digital synthesis; therefore, substan-

dards such as PAL 3.58 or NTSC 4.43 can also be

demodulated.

2.3.3. Chrominance Filter

The demodulation is followed by a lowpass filter for the

color difference signals for PAL/NTSC. SECAM re-

quires a modified lowpass function with bell-filter char-

acteristic. At the output of the lowpass filter, all luma

information is eliminated.

The lowpass filters are calculated in time multiplex for

the two color signals. Three bandwidth settings (nar-

row, normal, broad) are available for each standard

(see Fig. 2–5). For PAL/NTSC, a wide band chroma fil-

ter can be selected. This filter is intended for high

bandwidth chroma signals, e.g. a nonstandard wide

bandwidth S-VHS signal.

Luma / CVBS

1 H Delay

Notch

Filter

CrossSwitch

Luma

Chroma

Chroma

Fig. 2–4: Color decoder

IF Compensation

DC-Reject

MIXER

Lowpass Filter

Phase/Freq

Demodulator

ACC

ColorPLL/ColorACC

10

Micronas

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